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Twenty-five million years ago, as plate tectonics began to pull
apart eastern Africa, the landscape that would eventually be home
to the first humans began to take shape.

Now, new research says that landscape — and its lakes, rivers and
climate — may have looked quite different than scientists
thought.

The
East African Rift Valley, as the region is known, formed
where the Somalian and Nubian plates are pulling away from the
Arabian Plate. The eastern branch of the rift passes through
Ethiopia and Kenya, and the western branch forms a giant arc from
Uganda to Malawi.

The eastern branch formed around 25 to 30 million years ago,
whereas the western branch formed only 10 to 15 million years ago
— or at least that's what scientists thought. Now, new
evidence points to an earlier birth date for the western branch,
too. [ Have
There Always Been Continents? ]

"We now believe that the western portion of the rift formed about
25 million years ago, and is approximately as old as the eastern
part, instead of much younger as other studies have maintained,"
said Michael Gottfried, a geologist at Michigan State University
who co-authored the study.

"East Africa is traditionally where researchers go to try to
understand the evolution of primates and, eventually, humans,"
Gottfried told OurAmazingPlanet. "We're looking at how the
landscapes and environments in Africa changed during the run-up
to human evolution."

Rifting in the western branch 25 million years ago dramatically
changed how rivers flowed, where lakes formed and even the
region's climate patterns.

Geological evidence points to alternating wet and dry seasons,
and major rivers that flowed from south to north, Gottfried said.
These rivers likely drained into a large lake in the center of
the continent — Gottfried and his colleagues call it Paleo-lake
Congo — which no longer exists.

To piece together the story, Gottfried and his colleagues,
including Nancy Stevens, a paleontologist from Ohio University,
collected rock samples, zircons (a rare crystalline mineral that
can be used to pinpoint geological ages) and fossils, which they
correlated with similar specimens from other regions to determine
the ages of the rocks encasing them.

Rocks deposited more than 25 million years ago, before the
western branch began rifting, house fossils of more primitive,
extinct primates, Gottfried explained. Younger rocks have
primate fossils that are generally more like the primates living
in Africa today.

"We kind of think of that period of time as a transitional
period," he added.

Geologist Eric Roberts of Australia's James Cook University led
the team. Their findings were published March 26 in the journal
Nature Geoscience.